Metalized films are widely used in flexible packaging due to their high moisture barrier property. Resealable flexible packages for soft cookies, for example, often use metalized film laminates in order to preserve the cookies' soft texture. These packages are typically made by coating a pressure sensitive adhesive (PSA) onto an ink-coated polymeric film of a material such as polyethylene terephthalate (PET), passing the web into a convection drying oven, and then laminating it onto metalized film. The laminate is stored in a roll form, slit into a proper width, and then die cut to provide the resealable feature. The die cut rolls are transported by trucks to bakeries, where packages are filled with cookies and then heat-sealed.
FIG. 1 illustrates an ideal PSA failure mode for resealable flexible packaging. The ink-coated PET film is lifted from the metalized film with no PSA residue, no ink pick-up, and minimal noise. Because the PSA remains intact, the package also has the ability to be resealed. However, in practice, a major challenge with metalized film laminates is adhesive build-up. During storage and transport, heat and humidity cause the PSA to flow deeper into the metalized film surface. This can create an uneven peel when the package is opened, which often results in film tear. When the film is torn, the package is no longer resealable and hence the package contents may become stale over time.
One way to reduce PSA flow into the metalized film surface is by using a higher cohesive PSA. Higher cohesive PSA is usually selected from acrylic polymers having higher polar functional groups such as acrylic acid, methacrylic acid, or acrylonitrile as comonomers. Post addition with metallic crosslinkers such as zinc oxide and plasticizer may further reduce adhesive build-up over time on the laminate.
Because the PSA is located between the metalized and ink surfaces, the higher cohesive strength PSA is prone to lift ink from the PET film. This can result in ink-covered PSA layers being laid down on the metalized film surface after the package is opened. As a consequence, the resealability of the package is not functional. Aging behaviors of two currently used acrylic emulsion PSAs are shown in FIG. 2. As can be seen in FIG. 2, the peel strength of these adhesives tends to increase over time, particularly under high temperature and high humidity conditions. In some cases, ink-coated PSA is transferred onto the metalized film surface when the package is opened.
Therefore, a challenge remains that either the PSA is too soft, such that it migrates into the metalized film and causes film tear, or the PSA is too cohesively strong such that it causes ink to be lifted from the PET film, leading to PSA transfer onto the metalized film layer.
Blending acrylic emulsion PSAs is well known in the art. However, when blending a soft acrylic emulsion PSA with higher cohesive strength emulsion PSA, thermodynamics dictate that the higher cohesive PSA will reside against the ink surface due to its higher polarity, while the softer PSA will dry on the air/PSA interface due to its lower polarity. When the blended PSA is laminated with a metalized film, the high cohesive PSA tends to lie down against the ink, while the soft PSA tends to lie down against the metalized film. Laminates made by blending two acrylic emulsion PSAs with these characteristics undesirably exhibited both ink pick-up and metalized film tear.
There remains a need for improved pressure sensitive adhesives for resealable packaging, particularly PSAs which can simultaneously avoid the aforementioned problems of film tear and ink pick-up associated with currently available metalized film laminates.